Collaboration between antigen (Ag).specific Ic CD8 and CD4 T cells is critical for generating an effective ant-tumor I cell response, yet very little is known about where, when and how this occurs. We have developed a model that has allowed us to visualize Ag-specific T cells responding to tumor growing in vivo. The model employs adoptive transfer of CD8 T cells from 0T-I mice having a transgenic TCR specific for Kb/0VA257-264 into normal mice, and challenge with E.G7 tumor expressing OVA as a pseudo-tumor Ag. A contribution of host CD4 T cells to the response of the 0T-I cells has been demonstrated functionally, but these Ag-specific cells cannot be visualized. We are now developing a model that employs double adoptive transfer into normal mice of OT-1 CD8 T cells along with CD4 T cells from 0T-II mice having a transgenic TCR specific for I-Ad/OVA323-339. Using this model to visualize, quantitate and characterize the OVA-specific T cells during the course of a response to E.G7 tumor we will pursue four specific aims: (1) To describe the collaborative interactions that occur between Ag-specific CD8 and CD4 T cells during the course of a tumor-specific response. (2) To determine the role of CTLA-4 blockade in allowing effective anti-tumor T cell responses. (3) To examine CD8 and CD4 T cell collaboration in immunization for protection against tumor and the collaboration in the memory response. Novel immunization strategies based on recent work in our laboratory will be examined. (4) To characterize approaches for tumor immunotherapy by overcoming T cell tolerance. We anticipate that the results obtained in the proposed studies will contribute to a better basic understanding of the collaboration between CD4 and CD8 T cells in the immune response to cancer, and may suggest novel approaches for protective and therapeutic immunization strategies.